Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EST Tue Jan 18 2022

Valid 12Z Tue Jan 18 2022 - 12Z Fri Jan 21 2022

...Upper Midwest through the Great Lakes...
Days 1-3...

A clipper system will dive from North Dakota through the Upper
Great Lakes today and tonight, and then shift into Quebec and away
from the area on Wednesday /D2./ This system is fast moving and
has modest deep layer ascent. However, increasing WAA will enhance
moisture which will be squeezed out by the available forcing to
produce a swath of moderate to heavy snow from far NE Minnesota
through the U.P. of Michigan where WPC probabilities for more than
4 inches are as high as 90%. Within this larger swath, local
maxima in excess of 8 inches are likely on D1 in the Arrowhead
where onshore flow will transport enhanced moisture from Lake
Superior and upslope into the Iron Ranges.

As this low moves quickly to the east, in its wake will be
impressive CAA, with 850mb temps crashing to below -20C Wednesday
and Thursday. With persistent N/NW flow occurring as broad
troughing leads to cyclonic flow across the Lakes, periods of
moderate to heavy LES will be common both D2 and D3 in the
favorable N/NW snow belt areas. In some places the DGZ will be at
or below the surface, somewhat limiting the enhancement typical of
LES SLR. However, moderate to heavy accumulations exceeding 4
inches are likely according to WPC probabilities on D2 across
parts of the U.P. and in the Tug Hill Plateau. By D3, the heaviest
snowfall may fall across NW Indiana as fetch along the length of
Lake Michigan maximizes, aided by confluent land-lake breeze
interactions. There is some uncertainty into exactly where this
band will set up and how far onshore it can penetrate, but WPC
probabilities are moderate for 4+ inches near Gary, IN.


...Pacific Northwest and Northern Rockies...
Days 1-3...

A cold front trailing an Alberta Clipper moving across the Upper
Midwest will bank into the terrain of the Northern Rockies of MT,
with moist upslope flow producing moderate to heavy snowfall. Some
enhanced mid-level fgen collocated with at least weak upper
diffluence within the tail of a departing jet streak will aid in
ascent, and WPC probabilities on D1 are high for 4 inches in parts
of the Northern Rockies and into the Absarokas. Local
accumulations of 6-8 inches are likely in the higher terrain.

After a brief respite on D2, a modest shortwave will dig southeast
from the Gulf of Alaska around the periphery of an amplified
Pacific Ridge. While this feature has only weak amplitude, it will
be accompanied by increasing LFQ diffluence ahead of an
approaching 130+kt Pacific jet streak, and embedded within
increasingly confluent mid-level flow driving moisture onshore. As
this overlap of ascent shifts southeast, it will encounter PWs
that are progged to be +1.5 standard deviations above the climo
mean, supporting heavy snow in the terrain of the WA Cascades,
Northern Rockies, Sawtooth, and ranges of NW WY. Snow levels will
climb to as high as 6000 ft in the most impressive warm advection,
but will fall to 3000-4000 ft as height falls shift southeastward.
Above these levels are where the heaviest accumulations are
expected, and WPC probabilities for more than 6 inches are as high
as 40%. Although snow levels will be generally high, some light
accumulations are possible at the mountain passes including
Stevens, Lookout, and Marias Passes.


...Southeast...
Day 3...

An arctic cold front digging through the Southeast on Thursday
will stall near the Gulf and Southeast Coasts as moisture begins
to lift northward from the Gulf of Mexico. This moisture will be
drawn to the north within the RRQ of a strengthening and poleward
arcing jet streak over the Mid-Atlantic, aided by increasingly SW
mid-level flow downstream of a longwave trough axis behind the
surface front. This warm moist flow overrunning the cold front and
associated cold dome of high pressure may produce mixed
precipitation and freezing rain across the Carolinas and central
Gulf Coast states Thursday night. There remains considerable
uncertainty due to spread in the evolution of these features, but
it is becoming increasingly likely that significant wintry
precipitation will impact at least portions of the Southeast
beginning Thursday. WPC probabilities for 0.1" of freezing rain
are currently as high as 20% across central Louisiana and into the
Coastal Plain of NC.


Weiss